1. Field of the Invention
The present invention generally relates to image forming apparatuses such as copiers, facsimile machines, printers, and printing machines equipped with an LED-based drawing unit in which a plurality of units of LED (light emitting diode) arrays are arranged in the main scanning direction with staggered positioning in the sub scanning direction, and particularly relates to an image forming apparatus that can perform fine size adjustment by use of a line synchronizing signal of the main scanning direction.
2. Description of the Related Art
LED printers are provided with a drawing unit having a one-dimensional LED head, in which a plurality of LEDs (light emitting diodes) are arranged at predetermined pitches in the main scanning direction to form an array serving as a light emitting array unit. In response to image data to be drawn, the light emission of each LED of the LED head is controlled, thereby shining and focusing light on a photoconductor to draw an image. Such LED printers do not have a movable part like a polygon mirror used in laser printers, and are thus highly reliable. In the case of an extended width machine capable of producing a large size printout, with an advantage in that there is no need for optical space in which a light beam is scanned in the main scan direction as in the laser printers, the size of the machine can be reduced by providing an integrated LED head in which an LED array is integrated with optical devices such as SELFOC lens and the like. Because of this, LED printers are used in place of laser printers.
In the laser printers, one light source (laser diode) having an approximately 10-mW output is lighted to produce a light beam, which is then scanned by a polygon mirror, fθ lens, etc. In the LED printers, on the other hand, LEDs corresponding to respective pixels are arranged in the main scanning direction, and an electric current approximately ranging from a few mA to 10 mA is supplied to each LED to light it up. The control of data transfer and light emission is performed on a LED-specific basis.
As the size of printers and copiers becomes large, thus, the numbers of LEDs and driver ICs increase proportionately, resulting in a drop in the production yield and also an increase in the length of the unit. Under such circumstances, in order to maintain the precision of beam arrangement for drawing, there is a need to increase the precision of the components. This results in highly expensive unit prices for the components, compared to small-size printers and copiers.
Against this background, Patent Documents identified below disclose an apparatus in which a plurality of LED heads for use in inexpensive small-size printers or copiers are arranged in the axial direction of the photoconductor (main scanning direction), thereby providing for the maximum photoconductive width in the axial direction of the photoconductor to be exposed to light in a piecewise manner by these LED heads.    [Patent Document 1] Japanese Patent Application No. 2002-19178    [Patent Document 2] Japanese Patent Application No. 2002-283609
When arranging a plurality of LED array units in the main scanning direction with staggered positioning in the sub-scanning direction, Patent Document 1 corrects the displacement of image data caused by the staggered positioning of the LED array units in the sub-scanning direction in the units of lines, and further corrects a displacement less than one line width remaining after such correction. This provides an LED-based drawing unit capable of accurately correcting the displacement of image data in the sub-scanning direction.
When arranging a plurality of LED array units in the main scanning direction with staggered positioning in the sub-scanning direction, Patent Document 2 uses a division controlling unit to divide the image data to be transferred to the LED array units into data blocks corresponding to the respective LED array units, and uses a transfer timing controlling unit to transfer the divided image data blocks to the respective LED array units with respective time delays corresponding to the positional displacement of a focusing point in the rotational direction (travel direction) on the photoconductor. This provides an image forming apparatus capable of producing high-quality images despite the use of a plurality of LED array units and divisional exposure of the photoconductive layer of the photoconductor.
Patent Document 1 and Patent Document 2 teach correcting the displacement of image data caused by the staggered positioning of LED array units in the sub-scanning direction with reference to a case in which an image is formed without a size change by use of the apparatus with the staggered positioning of LED array units in the sub-scanning direction. These patent documents, however, are silent about the case in which an image is formed with fine size adjustment.
Conventional LED printers are provided with the function to perform fine size adjustment that enlarges or reduces the size of an image by the increments of 0.1% for the purpose of producing a large-size printout (forming an image on a long-size paper). Here, a supplemental description will be given of a process performed in the case of image formation without a size change. A plurality of LED array units are staggered as shown in FIG. 4. An image is formed without a size change by delaying image data in amount equal to the number of main scanning lines that corresponds to the distance in the sub-scanning direction between the reference LED array unit and an LED array unit of interest that is staggered relative to the reference LED array unit in the sub-scanning direction. If staggered positioning in the sub-scanning direction is equivalent to 412 main scanning lines, for example, the image data is output to an LED head 503 after waiting for 412 synchronizing signal detections since one line corresponds to one synchronizing signal. This produces an image having no displacement in the sub-scanning direction. When an image is to be formed with fine size adjustment, the interval of signal occurrence is shifted with respect to the signal (main scanning line synchronizing signal) for synchronizing with the displacement of an LED array unit in the sub-scanning direction, thereby achieving fine size adjustment by the increments of 0.1% for size enlargement or reduction. The use of such fine size adjustment for the forming of images, however, gives rise to a problem in that the shifting of the intervals of main scanning line synchronizing signals results in the displacement of image data.
Accordingly, with respect to an image forming apparatus in which a plurality of LED array units are arranged in the main scanning direction with staggered positioning in the sub-scanning direction, and in which proper image output is obtained by shifting the intervals of main scanning line synchronizing signals despite the staggered positioning of the LED array units in the sub-scanning direction, there is a need for an image forming apparatus that can obtain image data without displacement in the sub-scanning direction even in the case of image formation with fine size adjustment.